Device for compensating axial astigmatism of electron-optical systems



July 24, 1962 A. DELONG ETAL DEVICE FOR COMPENSATING AXIAL ASTIGMATISMOF ELECTRON-OPTICAL SYSTEMS Filed June 17, 1959 jjil 2 Sheets-Sheet 1INVENTORS firm/n je/o n July 24, 1962 A. DELONG ETAL DEVICE FORCOMPENSATING AXIAL ASTIGMATISM OF ELECTRON-OPTICAL SYSTEMS 2Sheets-Sheet 2,

Filed June 17, 1959 IN V EN TORS 9 jrahas .747 f/ver/bi/n v' WM Z 4United States Patent DEVICE FOR COMPENSATING AXIAL ASTIGMA- TISM 0FELECTRON-OPTICAL SYSTEMS Armin Dclong, Vladimir Drahos, and JanSpecialny, all of Brno, Czechoslovakia, assignors to Tesla, narodnipodnik, Prague, Czechoslovakia Filed June 17, 1959, Ser. No. 821,052 2Claims. (Cl. 25049.5)

This invention relates to a device for compensating axial astigmatism ofelectron-optical systems, particularly of electron microscopes.

The pole pieces or electrodes of the magnetic or electrostatic lensesused heretofore in electron-optical devices cannot be manufactured withsuch accuracy as is necessary to produce an exactly rotationallysymmetrical magnetic field. This is the cause of an optical defectcalled axial astigmatism, which defect decreases the resolving power ofelectron microscopes. In order to correct this optical defect it isknown to use so called stigmators located under the objective. Theyeither directly compensate the unsymmetry of the magnetic field of thelens or they compensate the astigmatic picture. The known devices ofthis kind used heretofore comprise electrostatic systems or permanentmagnets and have a complicated mechanism and uneasy handling.

The invention removes these inconveniences by a novel device forcompensating axial astigmatism of electronoptical systems with the aidof a magnetic field working as a cylindrical lens. The device accordingto the invention is arranged in such manner that the correcting systemcomprising at least four pole pieces of an electro-magnet is arranged inthe immediate neighbourhood of the corrected system, outside theevacuated space, is displaceable by mechanical means towards the opticalaxis of the corrected system and is electrically centerable by feedingthe exciting windings with alternating current.

An embodiment of the invention is shown in the accompanying drawings, inwhich:

FIG. 1 shows a magnetic stigmator in an axial longitudinal sectionalview,

FIG. 2 depicts the same stigmator in a cross-sectional v1ew,

FIG. 3 shows the arrangement of four pole pieces of a magnetic stigmatorwith an outline of the magnetic field, and

FIG. 4 shows a circuit diagram of a direct-current power source and analternating-current signal source and the interconnecting means by whichthe series-connected windings of the four pole pieces may be connectedto one or the other of the two sources.

An example of a device according to the invention and adapted tocompensate for axial astigmatism of electronoptical systems is themagnetic stigmator shown in FIGS. 1 and 2. Under the pole pieces 1 and 2is arranged a correcting system comprising four pole pieces 3 providedwith field coils 4 fixed within a tube -5 of a ferromagnetic material.The tube 5 forms a part of the magnetic circuit and also serves as ascreen for the electron beam. The field coils 4 are connected in seriesand are fed by a direct current supplied to terminals 6. The tube 5 isjoined with a flange 7 of insulating material. The main member 17contains the stigmator consisting of the parts 3, 4, 5, 6, 7, 8 joinedto form a unit which is rotatable round the optical axis within a rangeof :50 and controllable by a rod 8 linked with a graduated dial 9. Withthe aid of three set-screws 10 engaging a ring :11 the stigmator may bedisplaced in a plane perpendicular to the optical axis. The correctingsystem is separated from the evacuated space within the optical systemby an elastic packing 12 provided in a tubular member 13, the packing 12being pressed by a screw-nut 14 and a packing ring 3,046,397 PatentedJuly 24, 1962 ice '15 towards the surface of the tubular member 13. Thetubular member 13 is manufactured of a non-magnetic material and isfixed by three screws 16 tothe main member 17 provided with a projectingtube 18. The system of the stigmator is joined by three screws 19 withthe body 20 of the objective. The system may be displaced with the aidof set-screws 21 in respect to the projecting system by movements in aplane perpendicular to the optical axis, a vacuum-tight connection withthe optical system being secured by a packing ring 22.

The essential parts of the above described stigmator are four polepieces 3 arranged symmetrically round the optical axis. The magneticflux is produced by four field coils 4 arranged on tthe pole pieces 3.If the direction of the current in the field coils is chosen in such amanner that the pole pieces represent in succession north and southpoles, the magnetic field induced between said pole pieces has a formshown by field lines in FIG. 3. This magnetic field works like acombination of two cylindrical lenses, a condensing lens and adiverging-lens. The field is given by the following equations:

H kx and H -ky wherein:

H,, is the component of the magnetic field in the direction of the axisof x,

H is a component of the magnetic field in the direction of the axis ofy,

k is a constant coefficient.

The above described stigmator shown in FIGS. 1 and 2 corrects thepicture produced by the objective. The correcting field between the polepieces 3 should be as near as possible to the objective in order tolimit the additional distortion to a minimum. The intensity of thecorrecting field may be varied by changing the current in the fieldcoils 4. The desired direction of the correcting field is adjustable bythe lever 8 which carries with it the dial 9 provided with a scale forreading the angle of rotation of the stigmator. The accuracy of readingis better than :1. In order to reach a perfect correction it it isdesirable to shift the center of the correcting field so as to make thecenter coincide with the optical axis of the objective. Such positionmay be achieved by a mechanical centering of the stigmator by theset-screws 10. As an auxiliary meansure for centering the stigmator itpossible to feed the field coils by alternating current.

FIG. 4 shows a circuit diagram of a direct-current power source 48 andan alternating-current signal source 49 and interconnecting meansincluding a switch 45 by which the series-connected coils 4 may beconnected to one or the other of the two sources 48 and 49. Variableresistors 46 and 47 are provided to permit adjustment of the sourcevoltages to any desired values.

The stigmator according to the invention may be located between the polepieces thus making it possible to correct directly any unsymmetry of thefield of the objective instead of correcting only the astigmaticpicture. Even in such case the device may be arranged substantially inthe same manner as described above.

The stigmator may also correct the ellipticity of an illuminating beamor a distortion of dimensions of the picture in reflection electronmicroscopy.

It is an advantage of the invention that the system of the field coilsis located outside the evacuated space so that it is not necessary toarrange complicated vacuum bushings. The angle of correction is easilyadjustable by mechanical rotation of the whole system round the opticalaxis and the intensity of the correcting field may be adjusted bychanging the exciting current. The arrangement of the system off thepath of electrons avoids the formation of dirt whose electric chargedisturbs the symmetry of the field. The system is mechanicallycenterable in order to achieve an ideal correction the manufacture ofthe device is simpler than with the stigmators known heretofore.

What we claim is:

1. In combination in an apparatus for correcting the axial astigmatismof an electron-optical system which includes an evacuated chamber,magnetic lens means having a main axis along which an electron beam isto be projected, said magnetic lens means establishing an asymmetricalmagnetic field configuration with respect to said main axis, meanspositioned adjacent to said magnetic lens means for correcting theasymmetrical configuration of the magnetic field of said magnetic lensmeans, said correction means including four series-connectedelectromagnets disposed outside the evacuated spaced of theelectron-optical system in approximate quadrature relation with saidmain axis, said electromagnets having wind- 20 ings which are connectedin series in such a manner that the poles of said electromagnets whichface said axis and are opposite each other have identical magneticpolarities, variable direct-current power source means for energizingsaid four series-connected electromagnets, means for mechanicallyrotating said four series-connected electromagnets with respect to saidmain axis, and centering means for moving said four series-connectedelectromagnets in a plane perpendicular to said main axis.

2. A combination as in claim 1 wherein said centering means furtherincludes an alternating-current power source means, and means forinterrupting the energization of said four series-connectedelectromagnets by said variable direct-current power source means andfour energizing said electromagnets by said alternating-current means.

References Cited in the tile of this patent UNITED STATES PATENTS2,802,111 Reisner Aug. 6, 1957 2,910,603 Van Dorsten Oct. 27, 19592,914,675 Van Dorsten Nov. 24, 1959

